Combined cycle power plants have a gas turbine engine and a steam turbine engine. The gas turbine engine can be supplied with natural gas that is pre-heated (i.e. its temperature is adjusted before it is injected into the combustion chamber of the gas turbine engine) to optimize efficiency; usually pre-heating is achieved by heating the natural gas to a maximum allowable temperature using steam or feedwater from the steam cycle.
Flexibility with regards to the natural gas composition is an important feature for those combined cycle power plants that are supplied from different natural gas pipelines, from a LNG harbour or from a pipeline where additionally a natural gas treatment plant is installed.
In general when the natural gas source is changed, the natural gas composition is also changed. Change in natural gas composition can cause a change in the behavior of the combustion process. For example, a natural gas with high inert contents and hence a lower heating value requires an increased supply pressure; this may lead to changes in gas reactivity and mixing quality.
For this reason the composition of the natural gas supplied to a combined cycle power plant (in particular to a gas turbine of such a plant) is measured.
In order to measure the composition of the natural gas, different devices are known, namely:
Infrared analyzers, these devices allow measurement of the hydro carbons; in addition they have the advantage that their response is quite fast; this is useful because the natural gas supply (and thus the natural gas composition) can be changed on a weekly, daily or hourly basis or even much faster if for example in a LNP plant the higher hydro carbon trips according to the fuel supply conditions). Infrared analyzers have the disadvantage that they are not able to measure the natural gas content of nitrogen (N2, the N2 amount can be quite large in natural gas), because this compound does not respond to infrared radiation.
Chromatography analyzers, these devices can measure the hydro carbon content and also the nitrogen (N2) content of the natural gas. The disadvantage of chromatography analyzers is that their response is very slow, because it can take minutes for them to analyze the natural gas composition. Such a slow response can lead to instability of the combustion in the gas turbine engine.
The changes in natural gas composition could happen slowly if the switch of gas line is planned (usually around 5-30 minutes) or they could happen quickly if an unplanned event triggers a change of natural gas (usually around 30 seconds).